The present invention relates to a plane for manufacturing cement clinker. The plant consists of a preheater in which the raw materials are preheated, a calciner in which the raw material is calcined at a temperature of more than 800.degree. C. and where fuel and oxygen-containing gas are fed to the lowermost zone of the calciner, and a kiln in which the raw materials are burned and transformed into clinker.
In a conventional plant for manufacturing cement clinker it will be possible to substitute up to 20% of the primary fuel in a rotary kiln by low-cost waste fuel, such as rubber tyres, without sacrificing the quality of the finished cement product.
Normally, a maximum of 40% of the total fuel requirement of the cement plant is fired in the clinker kiln, with the remaining 60% being fired in the calciner. As a consequence hereof, in an ordinary kiln plant it will only be possible to substitute about 20% of the 40% fuel which is fired in the rotary kiln, i.e. approximately 8% of the total fuel rate, by waste fuel. Therefore, it would be advantageous if a greater part of the fuel burned in the calciner could be substituted by waste fuel.
However, there are certain limitations to the types of waste which can be advantageously burned in the calciner.
The temperature in the calciner will typically be within the range 850-950.degree. C., which means that there will not be any advantage in burning hazardous waste, the harmful substances of which have to be decomposed and neutralized during combustion, since this would require higher temperatures and a longer retention time than is available in the calciner.
Calciners are normally of the suspension type, which entails that the preheated air from the clinker cooler or from the kiln is carried via a duct and a flexible pipe bend up to the bottom of the calciner. If lumpy waste such as shredded rubber tyres is fed to the calciner, it will drop down into the pipe bend, causing it to be blocked. As a consequence hereof, calciners with a bottom being configured in this manner are not suitable for firing lumpy fuels.
It may also be difficult to utilize fuel which is difficult to ignite, e.g. certain coal types with a low content of volatile compounds, since the retention time in the calciner is relatively short and the temperature relatively low.
In Danish patent application No. 870/82 (Metallgesellschaft AG) is shown a fluid-bed calciner in which oxygen-containing fluidizing air is fed through the bottom of the calciner and where, furthermore, oxygen-containing secondary air can be fed horizontally to an area which extends from minimum 1 metre above the entry point of the fluidizing air and up to the lowermost 30% of the calciner. In this system at least 65% of the fuel which is used to cover the total heat requirement is fed to the calciner and at least 10% of the fuel is fed to the subsequent kiln. By generating a fluid bed in the calciner it is possible to achieve a high degree of calcination, even in case of variations in respect of particle size distribution. However, the fluidization means that a substantial amount of false air is introduced to the system, and this will have an adverse effect on the heat economy. The actual injection of the fluidizing air also requires a certain input of energy.
From Danish patent No. 170368 (F.L. Smidth & Co. A/S) it is known practice to feed lumpy material such as shredded automobile tyres to a decomposition chamber which is installed in connection with the calciner. According to this patent specification preheated or partially calcined raw meal is fed to the decomposition chamber so as to heat up the waste fuel. The temperature which it is desirable to achieve in the decomposition chamber must be sufficiently high to ensure that a pyrolysis process is carried out, but, on the other hand, it must not be so high as to result in granulation of the raw meal. The combustible gases from the decomposition chamber subsequently flow into the burning chamber which consists of the calciner itself, and the residual products are extracted from the bottom of the decomposition chamber and may be conveyed to the rotary kiln. However, the use of a decomposition chamber, in which an air deficit is being applied, will not allow the same amount of energy to be extracted from the waste in the calciner as can be obtained in connection with a complete combustion under conditions of sufficient oxygen supply.